I believe there's a difference between "competence" (I know how to say X) and "fluidity" (I can say X as easily as breathing).
You can learn the first on your own. Takes a lot of effort, sure, but it's doable.
I don't think you can learn fluidity outside of actually living in a country. You need to be bombarded daily with questions that you're forced to respond to. Even then, some people just don't make the effort, which is why you get people who have lived in a foreign country for decades and their language skills haven't improved.
My first three months in China were worth more than the preceding three years of self-study.
This is definitely true. I usually describe it as "language knowledge" vs "fluency" in my classes.
You can learn fluidity on your own, but it takes the right kind of practice. It's easy to think of learning a language as "study". You go through books learning words, phrases, verb endings etc. Most exercises in textbooks only care about you getting the right answer, not how quickly you do it.
Compare this to learning a musical instrument. There is some study needed, but realtime practice is the bulk of what you do. If you need 30 seconds to work out the next chord in a song then you can't play the piano. The gold standard is practicing with a metronome. The areas where you can't keep up are the areas where you need to practice.
I'm quite surprised that this idea of realtime practice is still so uncommon even though everyone has a smartphone nowadays. In the past, Pimsleur language courses put an emphasis on being able to answer in the language you are studying more or less straightaway.
I believe there's a difference between "competence" (I know how to say X) and "fluidity" (I can say X as easily as breathing).
You can learn the first on your own. Takes a lot of effort, sure, but it's doable.
I don't think you can learn fluidity outside of actually living in a country. You need to be bombarded daily with questions that you're forced to respond to. Even then, some people just don't make the effort, which is why you get people who have lived in a foreign country for decades and their language skills haven't improved.
My first three months in China were worth more than the preceding three years of self-study.